The present invention relates to a system for reducing the discharge of pollutants from underground gasoline storage tanks. The system is arranged to discharge pollutant free air when the pressure within the system reaches a predetermined level. Air to be discharged is separated from gasoline vapor within the storage system prior to its discharge.
U.S. Pat. No. 5,464,466, to Nanaji et al., describes a fuel storage tank vent filter system where a filter or fractionating membrane is used to capture pollutants from the vapor vented from the system's fuel storage tanks. A property of the membrane is that it will capture or collect selected pollutants including hydrocarbons. The captured pollutants are drawn from the membrane as a liquid and returned to the fuel storage tanks. The fractionating membrane comprises a plurality of stacked and bound thin sheets. Each sheet has a hole formed in its center to form an aperture in the stack extending axially from end to end. A perforated removal pipe must be positioned in the axial aperture to enable the captured vapors to be drawn out of the membrane under a vacuum created by a vacuum pump. The throughput of the system is limited because pollutant molecules, as opposed to air molecules, must be pulled through the fractionating membrane in liquid form. U.S. Pat. No. 5,571,310 discloses the use of such a membrane in an organic chemical vent filter system. Harmful volatile organic compounds (VOC's) are drawn through the membrane by using a vacuum pump to create a pressure drop of one atmosphere across the membrane. The pump is positioned between the membrane and the tanks, as opposed to between the membrane and the atmosphere.
These prior art systems are inadequate, however, because, to achieve adequate throughput, a substantial pressure drop, e.g., one atmosphere, must be created across the fractionating membrane. Further, the fractionating membrane of these prior art systems, and the associated hardware, is typically too large and costly for many applications. The pumping and fluid transfer system is likely to be more costly and difficult to assemble because of the relatively high levels of vacuum created in the system. Finally, the prior art systems do not expel substantially pollutant free air to the atmosphere. Rather, pressure within the tanks is reduced by merely condensing the pollutant vapors to liquid and returning them to the tanks. Accordingly, there is a need for a compact fuel storage system vent filter assembly that provides improved filtering and throughput at a competitive cost.